The cloud feedback in response to short-term climate variations is estimated from cloud measurements combined with off-line radiative transfer calculations. The cloud measurements are made by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite and cover the period 2000-2010. Low clouds provide a strong negative cloud feedback, mainly due to their impact in the shortwave (SW) portion of the spectrum. Mid-level clouds provide a positive net cloud feedback that is a combination of a positive SW feedback partially canceled by a negative feedback in the longwave (LW). High clouds have only a small impact on the net cloud feedback due to a close cancellation between large LW and SW cloud feedbacks. Segregating the clouds by optical depth, we find that the net cloud feedback is set by a positive cloud feedback due to reductions in the thickest clouds (mainly in the SW) and a cancelling negative feedback from increases in clouds with moderate optical depths (also mainly in the SW). The global average SW, LW, and net cloud feedbacks are +0.30±1.10, -0.46±0.74, and -0.16±0.83 W/m2/K, respectively. The SW feedback is consistent with previous work; the MODIS LW feedback is lower than previous calculations and there are reasons to suspect it may be biased low. Finally, it is shown that the apparently small control that global mean surface temperature exerts on clouds, which leads to the large uncertainty in the short-term cloud feedback, arises from statistically significant but offsetting relationships between individual cloud types and global mean surface temperature.

14 comments:

A.E. Dessler? I thought he had been the main proponent of positive feedback of clouds. Is this the same paper on hold that he and Roy Spencer were sparring each other on at http://www.drroyspencer.com/?s=dessler

A lot of work has gone into debunking this strange concept of cloud positive feedback. In tropical summer situations it is obvious to anyone, but easily confirmed by datalogging anywhere. Low cloud days are cooler. Low cloud at night means it may take longer for the surface temperature to drop, but the previous day's heat is still gone before sunrise. Convection makes a difference of course, but is apparently ignored in these radiative tranfer calculations.

Is this the same Dessler who does not understand heat & mass transfer and criticized a paper by Spncer and Braswell?Clouds affect (reduce or allow through) in coming solar radiation. They do not have a feedback because there is no feedback positive or negative. Clouds are a variable resistance to radiative heat flux. They are not a force or source of energy.

Jim Hansen, Roy Spencer (and IPCC et al) are all wrong in assuming the atmosphere would be isothermal without GHG.

They are also wrong in assuming that the Sun was capable of warming the surface of Venus, Earth or other planets to the observed temperature which is then maintained by back radiation being supposedly the only process that slows such surface cooling. They forgot that conduction and evaporation also decrease with a narrowing temperature gap.

The Second Law of Thermodynamics is stated (in Wikipedia “Laws of Thermodynamics”) thus …

“An isolated system, if not already in its state of thermodynamic equilibrium, spontaneously evolves towards it. Thermodynamic equilibrium has the greatest entropy amongst the states accessible to the system.”

If there were to be a sealed cylinder of air which was isothermal, then there would be an “ordered” state with more total energy (PE + KE) at the top. Hence this would not be an equilibrium state, because entropy could increase, and it must. There will only be equilibrium when the sum (PE+KE) is the same at all heights.

A vertical isothermal state in a gravitational field has less entropy than an isentropic state, the latter having maximum possible entropy, and thus being the equilibrium state as referred to in the Second Law of Thermodynamics as I quoted it above from the Wikipedia “Laws of Thermodynamics’ item. Hence a thermal gradient forms autonomously by diffusion at the molecular level.

Furthermore, any additional thermal energy deposited at the top can, and will, diffuse towards the bottom, creating a new equilibrium. This means there can be a heat transfer up the thermal gradient if that gradient is equal to or less in absolute magnitude than the normal equilibrium thermal gradient.

This is how energy absorbed in the Venus (or Earth) atmosphere at any altitude from any source, be it upwelling or downwelling radiation or latent heat release (on Earth) can flow towards the surface, heating the base of the atmosphere and subsequently heating the surface, or “supporting” its existing slightly warmer temperature by slowing the rate of cooling.